Fluidized bed reactors are effective means for generating heat and, in various forms, can carry out the processes of drying, roasting, calcining, heat treatment of solids with gases in the chemical, metallurgical, and other material processing fields, and the generation of hot gases, including steam, for use in driving electric power generation equipment, for process heat, for space heating or for other purposes. In reactors generating hot gases, air is passed through a bed of particulate material which includes a mixture of inert material and a fuel material such as coal, wood waste or other combustible materials. Where the combustion of bituminous or anthracite coal or other fuels containing a high sulfur component is undertaken, a material such as lime or limestone which will react with the sulfur released by combustion may be provided in the bed.
Fluidized bed reactors typically comprise a vessel having a substantially horizontal perforate plate; i.e., an air distributor or constriction plate, which supports a bed of particulate solids in the reaction chamber and separates the reaction chamber from a windbox below the plate. Combustion air is introduced into the windbox and passes through the air distributor in sufficient volume to achieve a gas velocity that expands or fluidizes the solids bed, suspending the particulate solids of the bed in the flowing air stream and imparting to the individual particles a continuous random motion. A fluidized bed in appearance and properties resembles a boiling liquid. Some important advantages of conducting a combustion reaction in a fluidized bed include the substantially uniform bed temperature, combustion at relatively low temperatures and a high heat transfer rate.
Commonly in such fluidized bed installations, the in-bed heat exchange tubes are disposed in a serpentine configuration with the lengths of the tubes extending in horizontal orientation; i.e., parallel to the upper surface of the fluidized bed. This kind of arrangement requires that the working fluid be pumped to assure satisfactory circulation of the water and steam through the system. The energy required to operate the pumps is a charge against the process. It is also well-known that horizontal in-bed tubes in serpentine configuration are susceptible to destructive erosion, especially at the return bends inherent in such arrays.
A disadvantage of typical fluidized bed boilers when compared with conventional boilers is due to the usual provision of a windbox below the combustion chamber. The windbox adds substantially to the total height of the unit so that additional clearance must be provided for prior art fluidized bed boilers.
Then, too, the flexibility in turn-down capability of known fluidized bed boilers has been limited. It is desirable at times to operate a boiler at substantially less than full capacity. While this can be achieved in conventional boilers by reducing the amount of air supplied, only a very modest change in capacity can be effected by this means in fluidized bed boilers, because the air supply must be maintained at a level sufficient to maintain the bed solids in a fluidized state. If the fluidized bed slumps, output of the unit ceases.